"IT'S stuck in my room somewhere," a 21-year-old Sydneysider, who didn't want to be named, told news.com.au. "There's a big pile of letters and it is probably in that."

As Australia embarks on the final days of the same-sex marriage campaign, this is the sentence that keeps Yes campaigners awake at night with the realisation that millions of votes may simply never get posted.

And yet, you could be forgiven for thinking the result was in the bag.

The most recent poll showed almost 60 per cent of those who had already voted had ticked "Yes". But this lead has steadily been decreasing over the course of the campaign.

In a tightening race, pro-same-sex marriage campaigners are becoming concerned. Not about No voters - but lazy Yes voters.

Their chief worry is the roughly two million people under the age of 35, most of who say they would vote Yes, who have yet to actually do so by casting their ballot. That apathy could tip the result towards No.

Yes campaigners are concerned younger people, inclined to vote Yes, could swing the result to No by failing to post their ballots. Picture: AFP PHOTO / PETER PARKS

On Saturday, about 15,000 people marched through the streets of Sydney in support of a Yes vote. On the same day, at least 150 protesters gathered outside the city's St Mary's Cathedral with the message "It's OK to vote No."

At the Yes rally, Greens Senator Lee Rhiannon said she feared the No vote could prevail and warned supporters not to take the poll lead for granted.

"That's great news but remember, last year, Brexit and the Trump wins defied the polls."

The Australian Bureau of Statistics (ABS), which is running the poll, has said all ballots should be in the postal system by this Friday, 27 October, to guarantee reaching the counting centres in each capital.

The ABS has confirmed that up until Friday 13 October, 11 million of the 16 million votes had been returned.

Proportionally, that means more Australians have voted in the postal survey than Americans in the election that saw Donald Trump become president or Irish that voted in that country's same-sex marriage referendum. But it still leaves up to five million votes up in the air.

Cory Bernardi says he expects the result to be much closer than predicted and many No voters could be telling pollsters they are voting Yes due to ‘political correctness’. Picture: AAP Image/MATT LOXTON.

But far from calling the race won, same-sex marriage campaigners have told news.com.au they're concerned those last votes could skew the result further towards No.

Indeed, Cory Bernardi, head of the Australian Conservatives and one of the chief No campaigners, has said "political correctness" was pushing people to pretend to pollsters they were in the Yes column when they were actually against a law change.

"I'm not even conceding defeat quite frankly," Bernardi told Sky News on Sunday. "I think there are a great many people who are deeply concerned about the potential consequences of changing the marriage act."

Even if the Newspoll is correct, the difference between the two camps - around two million - is still smaller than the number of votes yet to be cast. As many as 15 per cent of those have yet to make up their mind.

The big numbers for Yes could be also overstated by enthusiastic same-sex marriage supporters getting their votes in early.

Of particular concern is the high proportion of younger people, two thirds of whom favour Yes, failing to actually get to a post box.

The latest poll found that while 66 per cent of people aged 18-34 favoured Yes, only 57 per cent had posted their form.

In contrast, 74 per cent of over-65s had posted their ballot, the only demographic where the No view predominates.

According to the Australian Electoral Commission, some 4.3 million Australians under the age of 34 are eligible to vote.

With 43 per cent of those yet to post their ballots back, that leaves some 1.9 million total votes - of the up to five million outstanding - in the hands of younger Australians. Two-thirds of those votes could be for Yes.

Mr Brady encouraged under-35s to search the kitchen bench, the bottom of gym bags and under piles of supermarket catalogues to find their voting pack.

"We know there's an awful lot of young people that have either not voted yet or filled it in and not delivered it to the post box," he said,

A chief reason for the vote shortfall could be because younger people move more frequently and the ballot pack could be at a previous address or their parents' house.

"It's a good time to go home, get your dinner cooked for you and while you're there fill in the ballot and put it in the post box.

"There are still millions of votes out there and we want everyone's voice to be heard. This vote is about the dignity for people in our families, for friends, work colleagues and neighbour who simply want to be treated the same in law as everybody else in the communities they come from and love."

A concerted effort is on to get the Yes vote out. Picture: RICHARD JUPE

Monica Doumit, a spokeswoman for the Coalition for Marriage, said the No campaign wasn't letting up and would "be out there and talk to as many people as possible until the 7th of November".

While the ABS has been publishing a weekly running estimate of the ballots returned, the organisation said this information was not broken down by age range. This would only become clear after the result was declared on November 15.